This invention relates to a fuel dispenser and in particular but not exclusively to the type of dispenser common on service station fore-courts for dispensing diesel oil or petroleum for use in motor vehicles.
When fuel is dispensed it is nearly always metered for charging purposes. Because of the relatively high price and taxation levels of fuel, compared to other bulk liquids, stringent limits are laid down as to the accuracy with which the fuel must be metered. Contamination with gas, which term for the purposes of this specification including the claims includes vapour and/or air and other gases, results in inaccurate measurement of tile quantity of fuel dispensed. Contamination tends to occur during the pumping of fuel, especially diesel, whilst dispensing as this encourages vapourisation of the fuel and may induce air into the fuel through leaks in the system.
One way of overcoming the above problem is to incorporate a separator in a dispensing system upstream from the meter. This essentially comprises a closed tank in which the fuel passes through a wire gauze which tends to separate out the gas so that it can be vented off. Most fuel passes through to an outlet in the bottom of the separator and to the meter. The separator has to be of sufficient size to ensure adequate separation. This size is dependent upon the flow rate, viscosity and amount of contamination. These values can normally be calculated, but extremes of weather (fuel is more viscous in cold weather especially diesel oil), or faults in the system (for example air leaks in the suction pipe from the pump to a tank), cannot always be allowed for. Furthermore even under normal conditions the size of a separator can be prohibitive, especially in the case of more viscous fuels, and expensive centripetal separators may be necessary if metering of gas-free fuel is to be ensured.
Apparatus which attempts to solve the above problems is disclosed in Australian Patent No. 48522/72, in the name of Gilbarco Australia Limited. One embodiment of the invention disclosed in this earlier patent specification is illustrated in FIGS. 1 and 2 of the accompanying drawings.
In FIGS. 1 and 2 a dispensing system is illustrated which includes a gas separator 1 arranged such that gas entering the system is substantially drawn off through pipe 2, while substantially gas-free fuel passes via pipe 3 through valve 4 and meter 10 for dispensing. The pipe 2 passes through a venturi 5 and into a sump 6. Any fuel accumulating in sump 6 is returned by pipe 7 to the inlet of a pump unit (not shown). When gas-free fuel is passing along line 3 the venturi creates a reduction in pressure, hereinafter referred to as a vacuum, in chamber 8 shown in FIG. 2 which causes valve 4 to open. However should air be present, the vacuum is reduced and the valve 4 closes under the action of spring 9, shutting off the fuel for dispensing when gas is present, and therefore only permitting gas-free fuel to pass to the meter 10 which is monitored by a computer 11.
It has been found that employing a system as disclosed in FIGS. 1 and 2 is not an ideal solution to the problem as the system tends to hunt.